<div class="ui mini modal">
            <i class="close icon"></i>
            <div class="center aligned header">Введите логин и пароль</div>
            <div class="content">
                <form method="post" action="index.php" autocomplete="on" class="ui form centred-form">
                    <div class="field">
                        <label>Пользователь:</label>
                        <input type="text" autofocus='autofocus' id="user" required='required' placeholder="Пользователь">
                    </div>
                    <div class="field">
                        <label>Пароль:</label>
                        <input type="password" required='required' id="password" placeholder="Пароль">
                    </div>
                </form>
            </div>
            <div class="actions">
                <div class="answer"></div>
                <button class="ui blue button" name="do_login" id="logon_button" type="submit">Войти</button>
            </div>
        </div>

$("#logon_button").click(function() {

    var user = document.getElementById("user").value;
    var password = document.getElementById("password").value;

    $.post(
        "index.php", {
            user: user,
            password: password
        },
        onAjaxSuccess
    );

    function onAjaxSuccess(data) {

        if (data == "400") {
            $(".answer").html("Неверный логин или пароль");
        }else if(data == "200"){
            location.href = 'pages/admin.php';
        }
    }
});

if (isset($_POST['user'])) {

    $user = R::findOne("users", "login = ?", array($_POST['user']));

    if ($user) {

        if (password_verify($_POST['password'], $user->password)){
            $_SESSION['logged_user'] = $user->login;
            echo "200";

        }else{
            echo "400";
        }

    }
}